PRNDL & 4WD fuses blowing, went into limp mode. Any ideas?

[ShawnN]

99 K3500 CC LB 7.4L. I was driving this morning in heavy rain, running 55-60, when suddenly it acted like it went into passing gear. I lifted the throttle a bit but it never shifted back. I noticed the tach, speedo and gear selector indicator light all stopped working. I stopped and checked fuses and found that the fuses labelled PRNDL (Park, Reverse, Neutral, Drive and Low?) and 4WD had blown. I changed them and they continued to blow. I drove it to the shop in limp mode and he said he hadn't experienced that so I wanted to ask if anyone has an idea what the issue is. FYI, I left the 4WD fuse out the last time and the PRNDL fuse still blew.

 

[ShawnN]

Follow up on this. Here it is almost a year later and the truck is still in the shop. It is at a tranny shop (2 hours from my house) per the recommendation of the local shop owner, who personally knows the tranny shop guys and said they're good at fixing issues such as this. He works on it when the shop is slow. I call him about every two weeks and up to the most recent call he was optimistic he would find the problem; however, he is about to give up on it and I am NOT an electrical whiz. Here are things he has done that I'm aware of:

• I replaced the ignition switch harness, aftermarket brake controller and trailer wiring plug before taking it to him.
• Replaced TCU
• Replaced bad trans solenoids
• Swapped gauge cluster
• Moved trans power wire to different fuse/circuit
• Fixed bad wire to heater blower motor
• Fixed bad wire to distributor
• Fixed dual battery wiring issue
• Fixed hinky trailer wiring (previous owner fubar)
• Replaced wiring harnesses to rear of truck that had rubbed thru against frame, then replaced mating harness with badly corroded connector pins.
• Most recently said he was going to check some wires under the dash but no communication with him since then.

What really sucks is the truck only has about 90k miles. He said the tranny looks new inside and the motor runs great other than a miss under heavy load that I hope a tuneup will fix. The truck obviously sat a lot and I suspect it might have been flooded or frequently driven thru a creek (was obviously a farm truck) as the trailer plug had mud and water in it when I replaced it; this also explains the bad corrosion in the harness mentioned above. If anyone has any ideas PLEASE share them as I expect I may be getting it back soon still having the issue. About the only thing I can think to do is put a piece of wire in the fuse location and run it until the problem burns itself up...

Edit to add, early on he replaced a bad wire going to the transfer case which eliminated the 4wd fuse blowing.

 

[Schurkey]

Get a factory service manual, and follow the wiring diagram for the PRNDL fuse. Trace the actual wires on your vehicle until you find the problem.

My '97 manual does not show a PRNDL fuse, it has a "Trans" fuse #20 on the extreme left end of the dashboard, (I/P fuse block) which controls "PRNDL, auto trans, speedo, Check Gauges, tell-tale." It's hot in "Run", "Bulb Test" and "Start".

That fuse powers the tach as well as the speedo.

Apparently, my service manual has a missing section 8A-39, the transmission side of that fuse.

 

[ShawnN]

Get a factory service manual, and follow the wiring diagram for the PRNDL fuse. Trace the actual wires on your vehicle until you find the problem.

Thanks for the reply but that's what he's been doing with no luck.

ETA: I think it's the same fuse. It's been a long time since I've seen the truck but I believe it's fuse #20 as you mentioned. The guy working on it says he's traced pretty much every harness.